Method and structure for integrating capacitor-less memory cell with logic

ABSTRACT

Methods for fabricating integrated circuits include fabricating a logic device on a substrate, forming an intermediate semiconductor substrate on a surface of the logic device, and fabricating a capacitor-less memory cell on the intermediate semiconductor substrate. Integrated circuits with capacitor-less memory cells formed on a surface of a logic device are also disclosed, as are multi-core microprocessors including such integrated circuits.

This patent resulted from a continuation application of U.S. patentapplication Ser. No. 13/610,053, filed Sep. 11, 2012, entitled “MethodAnd Structure For Integrating Capacitor-less Memory Cell With Logic”,naming Gurtej S. Sandhu as inventor, which is a divisional applicationof U.S. patent application Ser. No. 12/338,404, filed Dec. 18, 2008, nowU.S. Pat. No. 8,278,167 B2, entitled “Method And Structure ForIntegrating Capacitor-less Memory Cell With Logic”, naming Gurtej S.Sandhu as inventor, the disclosures of which is hereby incorporated byreference.

TECHNICAL FIELD

The present invention, in various embodiments, relates generally tomethods for fabricating memory cells and logic devices on a commonsubstrate. More specifically, embodiments of the present inventioninclude a fabrication method in which a logic device is formed on anactive surface of a substrate, a semiconductor material is formed overthe logic device, and a so-called “capacitor-less” memory cell is formedon the semiconductor material. In addition, embodiments of the presentinvention comprise integrated circuits with at least one capacitor-lessmemory cell situated above a logic device, as well as multi-level arraysof memory cells situated above a substrate comprising logic.

BACKGROUND

Higher performance, lower cost, increased miniaturization of components,and greater packaging density of semiconductor devices are ongoing goalsof the electronics industry. Two significant classifications ofsemiconductor devices are logic and memory. Logic devices are used, incombinations conventionally termed microprocessors, primarily to processinformation. Memory devices, on the other hand, are used for informationstorage. Conventionally, while these two device types are found invirtually all electronic systems, such as computers and the like, theyhave been manufactured on separate integrated circuits and connectedonly at the card or board level. This has been due to differences inmanufacturing processes, cost considerations, economies of scale, andother difficulties in fabricating the different device structures on acommon substrate.

Trends in the semiconductor industry have led to making it moredesirable and feasible to blend memory and logic on the same integratedcircuit. Typically, in such structures a memory cell and a logic deviceare formed side-by-side in a single plane on a common substrate. Suchintegrated circuits are described in detail in, for example, U.S. Pat.No. 5,719,079 to Yoo et al. which is entitled Method of Making aSemiconductor Device Having High Density 4T SRAM in Logic with SalicideProcess, U.S. Pat. No. 6,353,269 to Huang which is entitled Method forMaking Cost-Effective Embedded DRAM Structures Compatible with LogicCircuit processing, U.S. Pat. No. 6,573,604 to Kajita which is entitledSemiconductor Device Carrying Memory and Logic Circuit on a Chip andMethod of Manufacturing the Same, and U.S. Patent ApplicationPublication No. 2008/0157162 to Doyle which is entitled Method ofCombining Floating Body Cell and Logic Transistors, the disclosures ofeach of which document is incorporated herein in its entirety by thisreference.

There are several drawbacks to these integrated circuits with memory andlogic positioned side-by-side on the same substrate. For example,state-of-the-art

multi-core microprocessors may have 4 or 16 processors on a singlesubstrate. Each processor requires that a significant portion of thearea, or “real estate” on the active surface of the substrate beoccupied by associated memory, consequently requiring a larger thandesirable semiconductor substrate or, stated another way, an undesirablylow number of processors on a given size substrate. Additionally, theremay be structural limitations for arranging the various processors onthe substrate so that each processor has adequate access to memorywithout unnecessarily consuming real estate or utilizing undesirablesignal lengths. Furthermore, while SRAM is conventionally the memoryintegrated with logic devices, SRAM structure does not provide goodcircuit density due to the number of required components per cell. TheSRAM fabrication process is compatible with that of logic devices;however, the overall process flow is inefficient.

In addition, conventional fabrication techniques which might otherwisebe used to combine memory with logic are impractical, due to the hightemperatures utilized in forming memory on a substrate alreadycomprising logic and metallization associated therewith.

Accordingly, there are needs for processes in which memories and logiccan be formed on a common substrate while minimizing the amount ofactive area on the substrate needed and maintaining efficiency of andaccessibility to memory by the logic.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, in which various features of embodiments of the presentinvention are depicted:

FIGS. 1 through 7 b are partial cross-sectional representations of anintegrated circuit under fabrication in accordance with embodiments ofthe present invention;

FIG. 8 is a partial cross-sectional representations of an integratedcircuit having a logic device and a superposed capacitor-less DRAMmemory cell in accordance with embodiments of the present invention;

FIG. 9 is a side schematic elevation of a logic device with two levelsof superposed capacitor-less DRAM memory cells thereover; and

FIG. 10 is a top schematic view of a multi-core processor in accordancewith embodiments of the present invention.

DETAILED DESCRIPTION

The present invention includes embodiments of integrated circuits havingcapacitor-less DRAM cells and logic devices formed on a common substrateand methods for fabricating such integrated circuits. Such methodsinclude the fabrication of a logic device on the active surface of asubstrate, formation of an intermediate semiconductor substrate over thesurface of the logic device, and fabrication of a capacitor-less DRAMcell on the intermediate semiconductor substrate above the logic device.

The following description provides specific details, such as materialtypes and processing conditions in order to provide a thoroughdescription of embodiments of the present invention. However, a personof ordinary skill in the art will understand that the embodiments of thepresent invention may be practiced without employing these specificdetails and in conjunction with conventional fabrication techniquesemployed in the industry. In addition, the description provided hereindoes not form a complete process flow for manufacturing a logic deviceor a capacitor-less DRAM cell, and the integrated circuit describedbelow does not form a complete semiconductor device. Only those processacts and structures necessary to understand the embodiments of thepresent invention are described in detail below. Additional acts to forma complete semiconductor device including the integrated circuitaccording to an embodiment of the invention may be performed byconventional techniques.

The materials described herein may be formed by any suitable techniqueincluding, but not limited to, spin coating, blanket coating, chemicalvapor deposition (“CVD”), plasma enhanced chemical vapor deposition(“PECVD”), atomic layer deposition (“ALD”), plasma enhanced ALD, orphysical vapor deposition (“PVD”). Alternatively, materials may be grownin situ. A technique suitable for depositing or growing a particularmaterial may be selected by a person of ordinary skill in the art. Whilethe materials described and illustrated herein may be formed as layers,the materials are not limited thereto and may be formed in otherthree-dimensional configurations.

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shown,by way of illustration, specific embodiments in which the invention maybe practiced. These embodiments are described in sufficient detail toenable a person of ordinary skill in the art to practice the invention.However, other embodiments may be utilized, and structural, logical, andelectrical changes may be made without departing from the scope of theinvention. The illustrations presented herein are not meant to be actualviews of any particular system, logic device, capacitor-less memorycell, or semiconductor device but are merely idealized representationswhich are employed to describe the present invention. The drawingspresented herein are not necessarily drawn to scale. Additionally,elements common between drawings may retain the same numericaldesignation.

FIGS. 1 through 7 b represent partial cross-sectional views of anintegrated circuit under fabrication in accordance with embodiments ofthe present invention. With reference to FIG. 1, an embodiment of aportion of an at least partially fabricated, or intermediate, logicdevice 106 is illustrated. Logic devices are well known in the art, sothe structural details of logic gates are omitted herein for clarity. Asimplified partially constructed logic device 106 is illustrated inFIG. 1. The logic device 106 includes a substrate 102 upon which thelogic device 106 is formed. The substrate 102 comprises a fabricationsubstrate, such as a full or partial wafer of semiconductor material(e.g., silicon, gallium arsenide, indium phosphide, etc), a full orpartial silicon-on-insulator (SOI) type substrate, such as asilicon-on-glass (SOG), silicon-on-ceramic (SOC), or silicon-on-sapphire(SOS) substrate, or any other known, suitable fabrication substrate. Asused herein, the term “wafer” includes conventional wafers as well asother bulk semiconductor substrates. The logic device 106 may becompletely fabricated or the logic device 106 may be partiallyfabricated. Partially fabricated logic device 106 may, as shown (not toscale), include logic L, shown schematically in broken lines, and levelsof metal wiring (two depicted) in the form of traces 108 comprising, forexample, copper or aluminum wiring surrounded by a dielectric material110 comprising, by way of non-limiting example, silicon dioxide,borophosphosilicate glass (BPSG), borosilicate glass (BSG),phosphosilicate glass (PSG) or the like. Logic device 106 has an upperor outer surface 112. Because the logic device 106 includes metal traces108, all subsequent processing acts should be conducted at a temperatureat or below about 400° C., so as to avoid thermal damage.

After the foregoing logic device 106 has been fabricated by employingconventional techniques, an intermediate semiconductor substrate, whichmay comprise silicon, may be formed over the surface of the logic device106. As a non-limiting example, the intermediate semiconductor substratemay be formed by a process described herein using a modification ofso-called SMART-CUT® technology. Such processes are described in detailin, for example, U.S. Pat. No. RE 39,484 to Bruel, U.S. Pat. No.6,303,468 to Aspar et al., U.S. Pat. No. 6,335,258 to Aspar et al., U.S.Pat. No. 6,756,286 to Moriceau et al., U.S. Pat. No. 6,809,044 to Asparet al., U.S. Pat. No. 6,946,365 to Aspar et al., and U.S. PatentApplication Publication No. 2006/0099776 to Dupont. However, otherprocesses suitable for manufacturing a semiconductor material on thesurface of a logic device may also be used, if sufficiently lowprocesses temperatures are maintained. In conventional implementation ofSMART-CUT® technology, donor and acceptor wafers are bonded togetherusing a high temperature anneal, on the order of about 1000° C. to about1300° C. However, the logic device 106 (FIG. 1), due to the presence ofmetal wiring in the form of traces 108, is unable to withstand exposureto such conventional, high temperature annealing used for wafer bondingwithout thermal damage. Accordingly, an additional plasma activation actmay be integrated into a conventional SMART-CUT® technology fabricationprocess to lower a required bonding temperature, as described in detailbelow.

FIG. 2 illustrates a donor wafer 114 comprising, for example, a siliconsubstrate. The donor wafer 114 has an attachment surface 119 which isimplanted, as described in the disclosures of the patent documents inthe preceding paragraph, with an atomic species 116, such as hydrogenions, ions of rare gases, also termed inert or noble gases, or ions offluorine, with a dose and energy to create an implanted zone 117, whichmay also be characterized as a transfer region, the inner boundary 118of which is shown in the donor wafer 114. The inner boundary 118 ofimplanted zone 117 lies substantially parallel to the attachment surface119 of the silicon donor wafer 114 and is at a predetermined depth whichis dependent on selected parameters of the atomic species implantprocess, as is well known to one of ordinary skill in the art. The innerboundary comprises a layer of microbubbles or microcavities comprisingthe implanted species, and provides a weakened structure within donorwafer 114. The donor wafer 114 is then thermally treated at atemperature above that at which implantation is effected, in accordancewith the disclosures of the patent documents in the preceding paragraph,to effect crystalline rearrangement in the wafer and coalescence of themicrobubbles or microcavities.

As shown in FIG. 3, the attachment surface 119 of the donor wafer 114 isthen exposed to a plasma 120 to form a plasma-activated silicon material122. The plasma 120 may comprise, for example, argon, argon and oxygen,argon and hydrogen, hydrogen, hydrogen and oxygen, nitrogen, ammonia(NH₄) and hydrogen/helium. The

plasma-activated silicon material surface, if a hydrogen plasma isemployed, exhibits a large number of dangling silicon-hydrogen bonds.The plasma-activated silicon surface increases the kinetics of asubsequent bonding act in the form of an oxide reaction with adjacentmaterial of the substrate 102 bearing logic device 106 (FIG. 1) due tothe increased mobility of the ionic species (for example, hydrogen)created on the attachment surface 119 of the donor wafer 114.Plasma-activated bonding is described in U.S. Pat. No. 6,180,496 toFarrens et al., assigned to Silicon Genesis Corporation.

As shown in FIG. 4, the plasma-treated silicon donor wafer 114 issuperposed onto the upper surface 112 of the logic device 106 with theplasma-activated silicon material 122 in contact with the upper surface112 of the logic device 106.

As shown in FIG. 5 the plasma-activated silicon material 122 on thedonor wafer 114 is bonded to the upper surface 112 of the dielectricmaterial 110 of logic device 106 by heating the assembly to atemperature of approximately 400° C. or less. Because the attachmentsurface 119 of the donor wafer 114 was exposed to the plasma 120 (FIG.3) to form a plasma-activated silicon material, the donor wafer 114 maybe bonded to the dielectric material 110 of logic device 106 at atemperature substantially lower than would be otherwise required using aconventional wafer bonding process. As noted above, plasma surfaceactivation prior to bonding increases the kinetics of an oxide reactioninitiated between the donor wafer 114 and the dielectric material 110 ofthe logic device 106 due to the increased mobility of the ionic speciescreated on the surface of the donor wafer 114. As a consequence, thedonor wafer 114 is bonded to the logic device 106 at a lower temperaturethan is possible using conventional techniques.

As shown in FIG. 6, the portion of the donor wafer 114 above (as thedrawing figure is oriented) the boundary 118 of implanted zone 117 iscleaved by applying a shearing force to the donor wafer 114, forming adetached donor wafer portion 125 and an intermediate silicon substrate124. The hydrogen or other ions implanted in implanted zone 117 to thedepth of inner boundary 118 makes the silicon in the thermally treateddonor wafer 114 susceptible to breakage along inner boundary 118 when ashear force is applied. The portion of the donor wafer 114 below theinner boundary 118, of a thickness, for example, of about 50 to about200 nanometers (about 500 Å to about 2000 Å), remains bonded to thelogic device 106 to become an intermediate silicon substrate 124. Thesurface 126 of the intermediate silicon substrate 124 exposed aftercleavage of the detached donor wafer portion 125 may be undesirablyrough and jagged. To remedy this deficiency, the exposed surface 126 ofthe intermediate silicon substrate 124 may be smoothed to a desireddegree in order to facilitate further processing as described below,according to techniques known in the art such as, for example, one ormore of grinding, wet etching, and chemical-mechanical polishing (CMP).

FIG. 7 a is an illustration of the logic device 106 with theintermediate silicon substrate 124 after exposed surface 126 has beensmoothed. Once the intermediate silicon substrate 124 is bonded and theexposed surface 126 thereof smoothed, then a memory cell may be formedthereon. For example, a capacitor-less DRAM memory cell, also known as afloating body memory cell, may be fabricated on intermediate siliconsubstrate 124 using conventional low temperature techniques so as not toadversely affect the logic device 106 underneath.

FIG. 7 b is an illustration of the fabrication of a capacitor-less DRAMmemory cell 128 within the intermediate silicon substrate 124. After theexposed surface 126 is smoothed, intermediate silicon substrate 124 ismasked and etched using conventional photolithographic techniques toform apertures surrounding the intended location of DRAM memory cell128, the apertures then being filled with a SiO_(x) material suitablefor use as an insulator material 130, which may also be termed adielectric material, such as, for example, SiO or SiO₂. Chemicalmechanical polishing may be used to remove excess insulator material 130from the surface 126 of the intermediate silicon substrate.

FIG. 8 illustrates, in an enlarged, simplified cross-sectional view, oneembodiment of a capacitor-less DRAM memory cell 128 fabricated withinthe intermediate silicon substrate 124. It will be understood that, inpractice, a plurality of such capacitor-less DRAM cells will befabricated above logic device 106 in association therewith.Capacitor-less memory cells are known in the art, and each may comprisea one transistor cell, wherein charge is stored in a channel and isrefreshed every few milliseconds. As a result, a large number of memorycells may be fabricated over a relatively small area on a substrate incomparison to the area consumed with conventional DRAM cells requiring adedicated capacitor structure. The capacitor-less memory cell 128includes an active region 132 surrounded on the sides by the insulatormaterial 130. The active region 132 may be formed from themonocrystalline silicon of the intermediate silicon substrate 124. Theentire depth of the intermediate silicon substrate 124 may be used toform the capacitor-less memory cell 128 as shown in FIG. 8, theunderlying dielectric material 110 on substrate 102 electricallyisolating active region 132 from below.

As shown in FIG. 8, high-k material for gate dielectric 136 is formed onthe location for active region 132. The material for gate dielectric 136has a dielectric constant that is greater than that of silicon dioxide.The gate dielectric 136 may be blanket-deposited by ALD techniques,formed of metal that is oxidized by a low temperature (e.g., 400° C. orless) oxidation process, or a combination thereof. Examples of asuitable material for high k gate dielectric 136 include hafniumsilicate, zirconium silicate, hafnium dioxide and zirconium dioxide. Ametal material for metal gate 138 is formed on the high k gatedeposition material 136. The metal gate 138 and underlying gatedielectric 136 may then be defined using conventional photolithographictechniques in combination with suitable etchants, as known to those ofordinary skill in the art.

Source and drain regions 134 may then be formed by doping exposedportions of the active region 132 flanking gate dielectric 136 and metalgate 138. The source and drain regions will be doped differently thanthe active region. For example, the active region may comprise p-dopedsilicon while the source and drain regions comprise n-doped silicon. Thedopants of the source and drain regions may be activated by using amicrowave anneal technique. Microwave anneal techniques are known in theart and may be used in an embodiment of the invention for activating thesource and drain regions at a temperature below 400° C. For example, thedoped source and drain regions 134 may be activated by exposing thoseregions to microwave radiation at about 350° C. Additional metal traces(not shown) connecting to memory cell 128 and further fabrication of thelogic device 106 may be completed on the assembly after thecapacitor-less DRAM memory cell 128 is formed.

In further embodiments, multiple capacitor-less DRAM memory cells may beformed in superimposition over a single logic device. In theseembodiments, a dielectric material, such as SiO_(x) may be formed over afirst capacitor-less memory cell and planarized as desired. Anothersilicon substrate and second capacitor-less memory cell may than berespectively disposed and formed on top of the first capacitor-lessmemory cell using techniques as described above. Such a structure isschematically illustrated in FIG. 9, wherein logic is designated as Land the two superposed levels of memory cells are respectivelydesignated MC1 and MC2. In further embodiments, a silicon substrate mayinclude multiple logic devices formed thereon, such as a multi-coremicroprocessor, with each logic device then carrying at least oneassociated capacitor-less DRAM memory cell thereover. Such a structureis schematically illustrated in FIG. 10, wherein each processor core isdesignated as PC in broken lines and the superposed groups of memorycells comprising a memory array associated therewith is designated MA.

Fabricating a memory cell on top of a logic device may enable adecreased signal length, on the order of angstroms (for example, withinthe range of about 100 Å to about 500 Å), from the logic device to theassociated, superimposed memory cell. This small signal length, incomparison to the micron-magnitude signal lengths between logic andmemory of a conventional processor, may improve signal response time inaddition to facilitating fabrication of the integrated circuit.Furthermore, forming the memory cell on top of the logic device maydecrease the volume of silicon needed for constructing the integratedcircuit. A bare silicon wafer substrate may be about 1000 Å to 5000 Åthick; however, each intermediate silicon layer as employed inembodiments of the invention may be only about 500 Å to 2000 Å thick.Thus, a smaller semiconductor substrate may be used for an array oflogic devices. Stated another way, multiple logic device arrays may beformed on the same wafer without requiring additional wafer real estateto carry the associated memory cells.

Although the foregoing description includes many specifics, these arenot limiting of the scope of the present invention but, merely, asproviding illustrations of some embodiments. Similarly, otherembodiments of the invention may be devised which are encompassed withinthe scope of the present invention. Features from different embodimentsmay be employed in combination. The scope of the invention is,therefore, indicated and limited only by the appended claims and theirlegal equivalents, rather than by the foregoing description. Alladditions, deletions and modifications to the invention as disclosedherein which fall within the meaning and scope of the claims are to beembraced thereby.

What is claimed is:
 1. A method for fabricating an integrated circuit,comprising: fabricating a wafer to comprise logic circuitry devices, thefabricated wafer having an outer surface; disposing a semiconductormaterial onto the outer surface of the wafer elevationally outward ofthe logic circuitry devices; and fabricating at least one capacitor-lessmemory cell on the semiconductor material elevationally outward of thelogic circuitry devices after disposing the semiconductor material ontothe outer surface of the wafer elevationally outward of the logiccircuitry devices.
 2. The method of claim 1, wherein fabricating a waferto comprise logic circuitry devices comprises fabricating the logiccircuitry devices onto a silicon substrate.
 3. The method of claim 2,wherein fabricating the wafer to comprise logic circuitry devicescomprises forming metal wiring over the silicon substrate.
 4. The methodof claim 3, further comprising disposing the metal wiring within adielectric material.
 5. The method of claim 1, wherein disposing asemiconductor material onto the outer surface of the wafer comprises:implanting ions to a depth into a donor silicon wafer; heat treating thedonor silicon wafer to a temperature higher than an implant temperature;exposing one side of the donor silicon wafer to a plasma; bonding theside of the donor silicon wafer exposed to the plasma to the outersurface of the wafer comprising logic; and removing silicon material atsubstantially the depth from an opposing side of the donor siliconwafer.
 6. The method of claim 5, wherein exposing the donor siliconwafer to a plasma comprises exposing the donor silicon wafer to a plasmacomprising at least one of argon, argon and oxygen, argon and hydrogen,hydrogen, hydrogen and oxygen, nitrogen, ammonia (NH₄) andhydrogen/helium.
 7. The method of claim 5, wherein bonding the donorsilicon wafer to the outer surface of the wafer comprising logiccomprises superposing the donor silicon wafer onto the outer surface ofthe wafer comprising logic and heating to a temperature of approximatelyabout 400° C. or less.
 8. The method of claim 5, further comprisingsmoothing a surface of the donor silicon wafer having the siliconmaterial removed therefrom.
 9. The method of claim 1, whereinfabricating at least one capacitor-less memory cell on the semiconductormaterial comprises: forming an active area from a portion of thesemiconductor material substantially surrounded on the sides and bottomby an insulator material; forming a high k gate dielectric and a metalgate on the active area; and forming a source region and a drain regionfrom the active area.
 10. The method of claim 9, wherein forming asource region and a drain region from the active area comprises: dopingthe active area in locations for a source region and a drain region;activating the dopant in the locations for the source region and thedrain region by microwave annealing at a temperature approximately 400°C. or less.
 11. The method of claim 10, wherein forming a high k gatedielectric on the active region comprises: forming a metal material onthe active area; oxidizing the metal material at a temperature ofapproximately 400° C. or less.
 12. The method of claim 1, wherein theouter surface is dielectric.
 13. The method of claim 1, whereindisposing a semiconductor material onto the outer surface of the wafercomprises bonding a donor wafer to the outer surface.
 14. The method ofclaim 1, wherein the fabricating of the memory cell comprises formingthe memory cell to comprise DRAM.
 15. The method of claim 1, wherein thefabricating of the memory cell comprises forming the memory cell tocomprise a floating body.
 16. The method of claim 1, wherein thefabricating of the memory cell comprises forming a transistor gatedielectric and a transistor gate over the semiconductor material, thegate dielectric physically contacting the semiconductor material. 17.The method of claim 1, wherein the fabricating of the memory cellcomprises forming a transistor gate over the semiconductor material andsource/drain regions in the semiconductor material.
 18. The method ofclaim 1, wherein the outer surface is planar.
 19. The method of claim 13comprising at least one of ion implanting, heat treating, and plasmatreating the donor wafer prior to the bonding.
 20. The method of claim19 comprising ion implanting the donor wafer prior to the bonding. 21.The method of claim 19 comprising heat treating the donor wafer prior tothe bonding.
 22. The method of claim 19 comprising plasma treating thedonor wafer prior to the bonding.
 23. The method of claim 19 comprisingion implanting, heat treating, and plasma treating the donor wafer priorto the bonding.
 24. The method of claim 13, wherein the donor wafercomprises silicon.
 25. A method of forming an integrated circuit,comprising: forming an implanted zone within a donor substrate having asurface to define a transfer region that includes the donor substratesurface; exposing the donor substrate surface to a plasma; bonding thedonor substrate surface to a logic device substrate; detaching the donorsubstrate along an inner boundary of the implanted zone and leaving thetransfer region bonded to the logic device substrate; and fabricating atleast one capacitor-less memory cell onto the transfer region after thedetaching of the donor substrate.
 26. The method of claim 25, whereinfabricating at least one capacitor-less memory cell onto the transferregion comprises: isolating an active region from a remainder of thetransfer region with an insulating region; forming a gate electrode onthe active region with a high k gate dielectric interposed therebetween;implanting an impurity in locations for a drain and a source in theactive region; and activating the impurity.
 27. The method of claim 26,wherein activating the impurity comprises microwave annealing.
 28. Themethod of claim 25, wherein forming an implanted zone in a donorsubstrate comprises implanting the donor substrate with hydrogen ions toa substantially uniform depth within the donor substrate.
 29. The methodof claim 25, wherein exposing the donor substrate surface to a plasmafurther comprises exposing the donor substrate surface to a plasma toprovide an activated ionic species on the donor substrate surface.